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Abstract:

A mobile terminal selects neighbor cell(s) to use in receiving a broadcast
or multicast by deriving an estimate of cell quality for each neighbor
cell; comparing the estimated quality with a minimum acceptable quality;
and choosing those neighbor cells having the highest quality from the
acceptable neighbor cells. The estimate of cell quality is based on a
parameter of the difference between a common pilot channel transmit power
and a secondary common control physical channel transmit power that is
transmitted to the mobile terminal on a multimedia broadcast multicast
control channel.

Claims:

1-7. (canceled)

8. A method of determining neighbor cell selection for a mobile terminal
in a broadcast or multicast, comprising:transmitting, to the mobile
terminal on a multimedia broadcast/multicast service control channel, a
parameter of the difference between a common pilot channel transmit power
and a secondary common control physical channel transmit power;deriving,
at the mobile terminal, an estimate of cell quality for each neighbor
cell;comparing the estimate of quality with a minimum acceptable quality
to identify acceptable neighbor cells; andchoosing selected neighbor
cells having highest quality from among the acceptable neighbor cells.

9. A method according to claim 8, wherein said deriving by the mobile
terminal includes:estimating a signal to interference ratio on the common
pilot channel; andscaling the signal to interference ratio according to
the parameter transmitted on the multimedia/broadcast multicast service
control channel.

10. A method according to claim 8, wherein said deriving the estimate of
cell quality by the mobile terminal includes obtaining a combination of
network power used to transmit a physical channel carrying a
broadcast/multicast data stream, derived from primary pilot physical
channel power, the parameter transmitted on the multimedia/broadcast
multicast service control channel, spreading factor, code puncturing
limit and transmission path loss between the mobile terminal and a
corresponding neighbor cell.

11. A method according to claim 10, further comprising estimating the
transmission path loss as a difference between a measured common pilot
reference channel received power and a common pilot channel transmit
power, scaled according to the parameter transmitted on the
multimedia/broadcast multicast service control channel.

12. A method according to claim 11, further comprising synchronizing the
selected neighbor cells with a serving cell so that respective
transmissions from the selected neighbor cells do not deviate in time
more than a size of a window in a radio link control combining function.

13. A method according to claim 12, wherein said choosing selects among
the acceptable neighbor cells so that a maximum timing difference between
any two cells in a cell group does not exceed a predetermined value.

14. A method according to claim 13,further comprising comparing a serving
cell MBMS traffic channel quality with a quality condition, andwherein
said choosing is performed when the serving cell multimedia broadcast
multicast service traffic channel quality fails the quality condition.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is based on and hereby claims priority to United
Kingdom Application No. 0413534.9 filed on Jun. 17, 2004 and United
Kingdom Application No. 0424510.6 filed Nov. 5, 2004, the contents of
which are hereby incorporated by reference.

BACKGROUND

[0002]Described below is a method of determining neighbor cell selection
for a mobile terminal in a broadcast/multicast.

[0003]The 3rd generation project partnership (3GPP) universal mobile
telecommunications system (UMTS) cellular radio system includes a
capability for the broadcast or multicast of data to multiple user
equipments (UEs). To increase the reliability of the data transmission
the UE is required to be able to receive transmissions from its primary
or serving cell and also, if necessary, from one or more neighbor cells.
The UE can then combine the signals that it receives from the plurality
of cells to produce a single data stream that has a lower rate of data
loss than would be obtained if decoding the primary cell alone. This
combining of MBMS traffic channel (MTCH) transmissions within a cell
group has been adopted for multimedia broadcast multicast service (MBMS)
and the provision of neighbor cell information on MBMS control channel
(MCCH) has been agreed.

[0004]However, as it is likely that, on any particular occasion, more
neighbor cells will transmit data than the UE is able to receive, it is
desirable for the UE to be able to select, from a number of neighbor
cells offered, only those which most effectively improve the resulting
single data stream.

SUMMARY

[0005]Described below is a method of determining neighbor cell selection
for a mobile terminal in a broadcast/multicast includes deriving an
estimate of cell quality for each neighbor cell; comparing the determined
quality with a minimum acceptable quality; and choosing those neighbor
cells having the highest quality from the acceptable offered neighbor
cells, wherein a parameter of the difference between a common pilot
channel (CPICH) transmit power and a secondary common control physical
channel (S-CCPCH) transmit power is transmitted to the mobile terminal on
a multimedia broadcast multicast service (MBMS) control channel, whereby
the mobile terminal derives the estimate of cell quality.

[0006]The mobile terminal determines which, if any, of the neighbor cells
will most enhance reliability of data transmission and chooses to accept
additional transmissions from those, using a new parameter transmitted to
the mobile terminal.

[0007]For the purpose of this application, reference to CPICH means
primary CPICH, or P-CPICH. Sometimes there is a secondary CPICH
available, but this would not always be present, so is not appropriate
for implementing this method.

[0008]The quality estimate may be the SIR (Signal to Interference ratio)
of the MTCH. In one embodiment, the mobile terminal estimates the SIR on
the CPICH and scales this ratio according to the transmitted parameter.
Alternatively, the mobile terminal derives the estimate of cell quality
from a combination of network power used to transmit a physical channel
carrying the broadcast/multicast data stream; spreading factor; code
puncturing limit and transmission path loss between the mobile terminal
and the neighbor cell.

[0009]The parameter enables the terminal to make a measurement of CPICH
signal to interference ratio (SIR), or power and adjust this value in
order that it is relevant to the MTCH.

[0010]Preferably, the transmission path loss is estimated as the
difference between a measured common pilot reference channel received
power and a common pilot channel transmit power, scaled according to the
parameter.

[0011]Preferably, the selected neighbor cells are synchronized with the
serving cell such that their respective transmissions do not deviate in
time more than the size of a window in a radio link control combining
function.

[0012]Preferably, the maximum timing difference between any two cells in a
cell group does not exceed a predetermined value.

[0014]An example of a method of determining neighbor cell selection for a
mobile terminal in a broadcast or multicast will now be described with
reference to the accompanying drawings in which:

[0015]FIG. 1 is a block diagram of a typical arrangement of a mobile
terminal and a number of cells for which the method described below can
be applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016]Reference will now be made in detail to the preferred embodiments,
examples of which are illustrated in the accompanying drawings, wherein
like reference numerals refer to like elements throughout.

[0017]The example of FIG. 1 illustrates a UE 1 surrounded by a number of
cells 2 to 7. Amongst these is a serving cell 2, through which the main
communications to the UE occur. However, to increase the reliability as
required by the 3GPP UMTS system, the UE also receives data from some
neighbor cells 3 to 7. The use of neighbor cells may occur in all
circumstances, or the UE may choose to select the neighbor cells, only if
the UE's own serving cell fails a quality target.

[0018]In the first case, the UE always selects and uses neighbor cells, to
within its capability. Selection of the best neighbor cell or cells is
completed as soon as possible, and is performed taking into account the
signalled parameter by which the CPICH to S-CCPCH power difference value
is provided. Alternatively, the UE does not select neighbor cells unless
the serving cell MTCH fails a quality condition. Based on known cell
selection procedures, the UE periodically measures only the serving cell
until the quality condition fails, after which all neighbor cells are
assessed. If the network controls the quality condition, a threshold
value is signalled as part of neighbor cell information on MCCH.

[0019]Assuming, for the purpose of this example, that the UE intends to
use data from one or more neighbor cells, then the UE needs to choose the
neighbor cells which will provide the most effective improvement, since
the UE is limited in the number of cells which it is able to accept
incoming data from, so the UE selects those which provide the highest
estimated quality

[0020]The selection of the cells that provide the lowest data error rate
is done by the UE estimating the quality of each cell from its knowledge
of the power with which the network will transmit the physical channel
that carries the CPICH and the power offset of the physical channel that
carries the MBMS data stream (MBMS S-CCPCH). Additional parameters, such
as the spreading factor that will be used on the MBMS S-CCPCH, the code
puncturing limit of the MBMS S-CCPCH and the transmission path loss
between the UE and the neighbor cell can also be used. The UE is able to
rank the cells in order of those likely to provide the highest quality
S-CCPCH signal.

[0021]SIR can be estimated by the UE measuring the received SIR of the
CPICH reference channel and scaling according to the CPICH transmit power
to MBMS transmit power offset.

[0022]Transmission path loss can be estimated by the UE measuring the
received power of the CPICH reference channel and subtracting the CPICH
transmit power to MBMS transmit power offset. Thus, if the UE has
available the value of CPICH Tx power-MBMS S-CCPCH Tx power for each
cell, then the UE can estimate the relative quality of each of the
neighbor cell transmissions and select those which are best. In practice,
the MBMS S-CCPCH Tx power tends to be greater than the CPICH Tx power, so
to maintain a positive result the network calculates and transmits the
parameter as MBMS S-CCPCH Tx power-CPICH Tx power for each neighbor cell.
One option is for these parameters to be transmitted with other neighbor
cell information on the MBMS Control Channel (MCCH).

[0023]Given that selective combining of MTCH transmissions made within
cell groups forms part of MBMS operation, it can be expected that
specified minimum UE capabilities will identify a minimum number of
neighbor cells that a UE must be capable of receiving simultaneously at
specified rates or transmission time interval (TTI) lengths. Since
generally this is likely to be less than the number of neighbor cells
that are identified in the MCCH transmissions the UE has to determine a
subset that it can receive. It is assumed that the UE always receives the
MTCH transmission of its serving cell and that the neighbor cells will be
a subset of the cells identified in system information block (SIB) 11/12.

[0024]A further improvement is to adopt a default value of zero and apply
a cell specific offset. Thus, in this case the parameter will be S-CCPCH
Tx power-CPICH Tx power-cell specific offset=0. This means that if a
parameter is not signalled a value of zero can be assumed. If the offset
is set to the most common value for S-CCPCH Tx power-CPICH Tx power
amongst all neighbor cells, then this can be used to minimize the number
of neighbor cells for which the parameter has to be signalled, thereby
reducing the signalling overhead. In addition, the offset reduces the
parameter dynamic range.

[0025]It is possible to use an offset because the UE only needs to select
the best neighbor cells, i.e. to place them in rank order as referred to
above. For each neighbor cell, i, only the value of ri=(S-CCPCHi
Tx-P-CPICHi Tx-offset)+measured P-CPICHi needs to be formed to be able to
place the neighbor cells in rank order and choose the best, since the
offset is common to all neighbor cells and consequently does not affect
their rank order.

[0026]Selecting which neighbor cells to receive is carried out
autonomously in the UE with an assumption of two constraints. The first
of these is that the UE should select only neighbor cells such that the
timing offset between the serving and neighbor cells does not exceed a
threshold. The second assumption is that the neighbor cells should
provide the best, or at least within a measure of the best, quality of
all neighbor cells that are available. The quality measure may be based
on SIR or block error rate (BLER) estimated from MBMS transmit power,
spreading factor and code rate.

[0027]The timing offset requires UTRAN input, but the quality control can
be completed either using rules specified by standards using parameters
provided by the network, or using UE specific rules that are tested via
performance testing. Synchronization of the serving and all selected
neighbor cells is such that their transmissions do not deviate in time by
more than the size of the window that is used by the RLC selective
combining function. To achieve this requirement the transmissions within
a cell group are controlled so that the maximum timing difference between
any two cells in a cell group does not exceed a known value. This maximum
timing difference, translated into numbers of RLC protocol data units
(PDUs), is transmitted on MCCH and used by the UE as the selective
combining window size. Timing accuracy can then be excluded from any
neighbor cell selection process. The maximum window size is defined by
the minimum UE capability.

[0028]Other modes of operation are possible, such as identifying for each
neighbor cell a maximum timing deviation and arranging for the UE to
select only those cells that are within its capability to receive. This
allows greater flexibility in UE capability and cell transmission timing,
but such flexibility is not always necessary or desirable when the added
complexity of such a mode is taken into account.

[0029]The UE autonomously selects the neighbor cells that it uses and this
selection must meet the necessary quality requirements. The UE makes an
initial selection whenever a bearer is established and whenever it
performs cell reselection whilst receiving a point to multipoint (p-t-m)
bearer. In each case there may be a delay whilst the UE makes
measurements of neighbor cells and makes the selection and consequently
there may be a period where it receives only the serving cell. In the
case of cell reselection the delay may include the time taken to receive
MCCH and possibly SIB 11/12.

[0030]Using known measurement practice for cell reselection for the
neighbor cell selection, means that the UE completes measurements for all
neighbor cells that are listed in MCCH before deciding on a correct
choice. Alternatively, the UE can measure until a suitable cell is found,
then accept that one only. If the UE can accept more than one neighbor
cell, this process continues until the desired number have been accepted.

[0031]Where a UE is using neighbor cells then, from time to time, it is
necessary to re-assess the appropriateness of the chosen neighbor cells,
which can be done in a number of ways. One option is that the neighbor
cells are replaced only if their quality falls below a given threshold.
The UE periodically measures only the neighbor cells in use until the
predetermined quality condition fails, after which an assessment of all
cells is made. If the network controls the quality condition, a threshold
value can be signalled as part of neighbor cell information on MCCH.

[0032]An alternative method is that all neighbor cells are assessed
periodically and the best adopted for use. No control parameters are
required by this method, but the minimum period between periodic updates
is specified. Changing the cells from which transmissions are made should
not disrupt operation of the RLC selective combining function, so it is
not expected that there would be significant costs from changing neighbor
cells.

[0033]Control over the way in which neighbor cells are
selected/re-selected could be set out as part of the standard or left to
each service provider to determine. If the control formed part of the
standard and specified rules for UE neighbor cell selection and
reselection, then selection/reselection trigger parameters may be
included. In either case, relative S-CCPCH and CPICH Tx power information
may be required from the UTRAN to allow the UE to compare cells e.g. if
CPICH RSCP were to be used as a comparison measure.

[0034]A description has been provided with particular reference to
preferred embodiments thereof and examples, but it will be understood
that variations and modifications can be effected within the spirit and
scope of the claims which may include the phrase "at least one of A, B
and C" as an alternative expression that means one or more of A, B and C
may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d
870, 69 USPQ2d 1865 (Fed. Cir. 2004).